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In 2012, China's first national shale gas demonstrations areas were set up in the Sichuan Basin. After 10 years' construction and practice, the giant marine shale gas area of 10 × 1012 m3 level is built up in the Sichuan Basin, and shale gas steps into a new stage of large-scale benefit exploration and development. In order to systematically summarize the achievements in shale gas exploration and development and provide guidance and reference for the exploration and development of deep shale gas and shale oil & gas in other areas, this paper systematically summarizes the main characteristics, development models and key identification and evaluation technologies for quality reservoirs of Wufeng Formation–Longmaxi Formation shale gas by analyzing the electric property, lithofacies, reservoir parameters and microscopic porosity of key wells in the basin. And the following research results are obtained. First, biogenetic siliceous shale, calcareous shale and mixed shale are main lithofacies types in quality shale gas reservoirs, and they are formed in the environment of semi-deep and deep water continental shelf. Their lateral distribution is controlled by the paleogeomorphology and their vertical development is influenced by provenance, redox condition and paleo productivity. They are 25–90 m thick. Second, in the quality shale gas reservoirs develop organic pores, inorganic pore and microfractures (including lamina/bedding fractures), among which, organic pore is one of the main reservoir spaces and microfracture is not only indispensable reservoir space, but also production pathway. The reservoir space of shale gas is overall micro-nano pore, and macropores play an important role in shale gas enrichment. Third, three development models of quality reservoir are established, including sedimentary type, diagenesis type and reworking type. The sedimentary type is the foundation. Multiple quality reservoirs are developed in the high U/Th interval of graptolite belt at the bottom of Longmaxi Formation, and their thickness is mainly controlled by paleogeomorphology and especially greater in the depression area. The diagenesis type is divided into three forms, i.e., syngenetic-early diagenetic rigid support, middle-late diagenetic mineral-organic matter transformation, and overpressure relief compaction. The reworking type is dominated by quality reservoirs with microfractures. Fourth, the core technologies for identifying and evaluating quality shale gas reservoir include large-size core and rock slice observation, high-accuracy rock mineral identification, experimental gas content test and simulation, SEM microscopic characterization, 3D microscopic pore reconstruction, comprehensive geophysical interpretation and prediction and big data analysis. In conclusion, nearly 10 years' research and practice achievements in demonstration area construction can deepen the understanding on domestic quality shale gas reservoirs, promote the effective development of the theories and technologies related to shale gas reservoirs, improve the prediction accuracy of shale gas sweet spot zones/intervals, and expand the shale gas exploration and development achievements of demonstration areas.

The exploration and development theory and technology of marine shale gas with shallow burial depth of 3500 m in South China has become mature after more than 10 years of practice. In order to continuously promote the exploration and development of shale gas in China, based on the research results of many scholars, combined with production practice, the main characteristics of marine shale gas in China and the main theory and technology of exploration and development are further summarized. The results show that: (1) The basic characteristics of marine shale gas in southern China are marine deep-water shelf deposition, and organic-rich shale is distributed continuously in a large area; The organic matter is stored in situ after high thermal maturity gas generation, and the gas content of shale is affected by late tectonic activity; The reservoir space is dominated by nano-scale pores, and the reservoir is super-tight and ultra-low permeability, which has no natural productivity without well stimulation; Bedding and natural fracture affect the productivity of shale gas wells; The mechanical properties of shale rock determine the effect of horizontal wells. In the early stage of well production, the production rate is high and the decline is fast, while in the middle and late stage, the rate is low but the production cycle is long. (2) Based on the above characteristics, the theory of “sweet spot” and “sweet interval” of shale gas and the theory of effective development of artificial gas reservoir are established, that is, the sedimentary and tectonic interaction forms the “sweet area” or “sweet spot” of marine shale gas. Gas productivity is determined by geological and engineering factors, so the “sweet spot” must be found to develop. By constructing artificial gas reservoir through “making an artificial high permeability area, and reconstructing a seepage field”, the effective development of ultra-tight and low permeability shale gas reservoirs can be realized. It is the most effective technique for shale gas development to accurately create “transparent geological body” of reservoirs based on geology-engineering integrated evaluation technology and establish artificial gas reservoir through multi-stage fracturing technology. (3) The overpressure area in the southern Sichuan Basin is the “sweet area” for shale gas development. The “sweet interval” of the lower Silurian Longmaxi Formation with a thickness of 3–5 m and high brittle-rich organic matter is the optimal “golden target” for horizontal wells. (4) The development leapfrog from the first generation to the second generation has been realized by the key technology of multi-stage fracturing of horizontal wells. The EUR per well of Upper Ordovician Wufeng Formation and Longmaxi Formation in southern Sichuan has increased from 0.5 × 108 m3 in the initial stage to 1.0 × 108–1.2 × 108 m3 at present with a buried depth of shallower than 3500 m. (5) The effective area favorable for shale gas development in southern Sichuan is 2.0 × 104 km2, and it is estimated that the proven geological reserves of shale gas can reach 10 × 1012 m3. It is preliminarily predicted that the shale gas production in China will exceed 300 × 108 m3 in 2025 and reach 400 × 108 m3 in 2035.

Tremendous reserves of shale gas resources exhibit broad development prospects in southern Sichuan, China. ”Horizontal well + hydraulic fracturing” is taken as the principal means of shale gas reservoirs development. The design of hydraulic fracturing parameters is of great importance for the formation and extension of artificial fractures. With Well Area X of the shale gas reservoir from the Ordovician Wufeng Formation to the Silurian Longmaxi Formation in Luzhou, southern Sichuan, China, as an example. This paper identifies the artificial fracture-related characterization parameters, presents a detailed portrayal of the spatial characteristics of the fracture network, and optimizes the 4 key parameters of fracturing construction (i.e., injection rate, cluster spacing, fluid injection intensity, and proppant injection intensity) based on the geology–engineering integration fracturing simulation by building a regional 3D geological model. The result suggests that optimal injection rate, optimal cluster spacing, optimal fluid injection intensity, and optimal proppant injection intensity exist under certain geological characteristic parameters. According to the 3D geological model and simulation results of the study area, the optimal injection rate, optimal cluster spacing, optimal fluid injection intensity, and optimal proppant injection intensity are 14 m3/min, 10 m, 30 m3/m, and 2.5 t/m, respectively. The 3D geological model-based method for optimizing the fracturing parameters of shale gas reservoirs using geology–engineering integration can also applies to identifying the optimal fracturing parameters of other shale gas or tight oil well areas.

As shale gas development is advancing continuously and rapidly, how to deeply analyze the production performance of shale gas wells and evaluate their production characteristics has become an urgent problem in the evaluation of shale gas productivity construction zone, the formulation of new area development scheme and the preparation of planning program. Some scholars have applied the Logical Growth Model (LGM) in the production decline analysis of unconventional gas wells, but the influences of shale gas reservoir and development characteristics are not taken into consideration. Therefore, this method still has some space of further development and improvement. In this paper, a Logistic Growth Model considering shale gas reservoirs and development characteristics (RB-LGM) was established based on the previous research results. Then, it was applied to the shale gas development wells in the Changning Block of the Sichuan Basin to analyze their production performance, and the analysis results were compared with the fitting and prediction results provided by the Arps hyperbolic decline model. Finally, the optimal well spacing of horizontal wells was determined using RB-LGM. And the following research results were obtained. First, shale gas is produced by deploying horizontal wells in the clustered pattern in a large number, so on the basis of LGM, RB-LGM takes shale gas reservoir parameters (thickness, shale density, gas content) and development parameters (horizontal section length, well spacing and recovery factor) as the logic control factors of horizontal-well gas production fitting, so that the production prediction result of gas well is more reasonable. Second, RB-LGM can not only well fit the early production data of gas well, but ensure the convergence of the later prediction results under the control of logical conditions. Third, RB-LGM takes into account the influence of shale gas reservoir and development characteristics so as to optimize the horizontal well pattern and analyze the change trend of reservoir parameters in the development area through data inversion.

The flow of shale gas in nano scale pores is affected by multiple physical phenomena. At present, the influence of multiple physical phenomena on the transport mechanism of gas in nano-pores is not clear, and a unified mathematical model to describe these multiple physical phenomena is still not available. In this paper, an apparent permeability model was established, after comprehensively considering three gas flow mechanisms in shale matrix organic pores, including viscous slippage Flow, Knudsen diffusion and surface diffusion of adsorbed gas, and real gas effect and confinement effect, and at the same time considering the effects of matrix shrinkage, stress sensitivity, adsorption layer thinning, confinement effect and real gas effect on pore radius. The contribution of three flow mechanisms to apparent permeability under different pore pressure and pore size is analyzed. The effects of adsorption layer thinning, stress sensitivity, matrix shrinkage effect, real gas effect and confinement effect on apparent permeability were also systematically analyzed. The results show that the apparent permeability first decreases and then increases with the decrease of pore pressure. With the decrease of pore pressure, matrix shrinkage, Knudsen diffusion, slippage effect and surface diffusion effect increase gradually. These four effects will not only make up for the permeability loss caused by stress sensitivity and adsorption layer, but also significantly increase the permeability. With the decrease of pore radius, the contribution of slippage flow decreases, and the contributions of Knudsen diffusion and surface diffusion increase gradually. With the decrease of pore radius and the increase of pore pressure, the influence of real gas effect and confinement effect on permeability increases significantly. Considering real gas and confinement effect, the apparent permeability of pores with radius of 5 nm is increased by 13.2%, and the apparent permeability of pores with radius of 1 nm is increased by 61.3%. The apparent permeability model obtained in this paper can provide a theoretical basis for more accurate measurement of permeability of shale matrix and accurate evaluation of productivity of shale gas horizontal wells.

As an important type of unconventional hydrocarbon, tight sandstone oil has great present and future resource potential. Reservoir quality evaluation is the basis of tight sandstone oil development. A comprehensive evaluation approach based on the gray correlation algorithm is established to effectively assess tight sandstone reservoir quality. Seven tight sandstone samples from the Chang 6 reservoir in the W area of the AS oilfield in the Ordos Basin are employed. First, the petrological and physical characteristics of the study area reservoir are briefly discussed through thin section observations, electron microscopy analysis, core physical property tests, and whole-rock and clay mineral content experiments. Second, the pore type, throat type and pore and throat combination characteristics are described from casting thin sections and scanning electron microscopy. Third, high-pressure mercury injection and nitrogen adsorption experiments are optimized to evaluate the characteristic parameters of pore throat distribution, micro- and nanopore throat frequency, permeability contribution and volume continuous distribution characteristics to quantitatively characterize the reservoir micro- and nanopores and throats. Then, the effective pore throat frequency specific gravity parameter of movable oil and the irreducible oil pore throat volume specific gravity parameter are introduced and combined with the reservoir physical properties, multipoint Brunauer-Emmett-Teller (BET) specific surface area, displacement pressure, maximum mercury saturation and mercury withdrawal efficiency parameters as the basic parameters for evaluation of tight sandstone reservoir quality. Finally, the weight coefficient of each parameter is calculated by the gray correlation method, and a reservoir comprehensive evaluation indicator (RCEI) is designed. The results show that the study area is dominated by types II and III tight sandstone reservoirs. In addition, the research method in this paper can be further extended to the evaluation of shale gas and other unconventional reservoirs after appropriate modification.

Three-dimensional (3D) geological property modeling is used to quantitatively characterize various geological attributes in 3D space based on geostatistics with the help of computer visualization technology, and the results are often stored in grid data. The 3D geological property modeling includes two main components, grid model generation and property interpolation. In this review article, the existing grid generation methods are systematically investigated, and both traditional and multiple-point geostatistical algorithms involved in interpolation methods are comprehensively analyzed. It is shown that considering the numerical simulation of oil reservoirs, the orthogonal hexahedral grid remains the most suitable grid model for simulations in petroleum exploration and development. For the interpolation methods aspect, most geological phenomena are nonstationary, to simulate various types of reservoirs; the main development trends are increasing geological constraints and reducing the limitation of stationarity. Both methods have certain constraints, and the multiscale problem of multiple-point geostatistics poses a main challenge to the field. In addition, the deep-learning based method is a new trend in geological property modeling.

In recent years, big data and artificial intelligence technology have developed rapidly and are now widely used in fields of geophysics, well logging, and well test analysis in the exploration and development of oil and gas. The development of shale gas requires a large number of production wells, so big data and artificial intelligence technology have inherent advantages for evaluating the productivity of gas wells and analyzing the influencing factors for a whole development block. To this end, this paper combines the BP neural network algorithm with random probability analysis to establish a big data method for analyzing the influencing factors on the productivity of shale gas wells, using artificial intelligence and in-depth extraction of relevant information to reduce the unstable results from single-factor statistical analysis and the BP neural network. We have modeled and analyzed our model with a large amount of data. Under standard well conditions, the influences of geological and engineering factors on the productivity of a gas well can be converted to the same scale for comparison. This can more intuitively and quantitatively reflect the influences of different factors on gas well productivity. Taking 100 production wells in the Changning shale gas block as a case, random BP neural network analysis shows that maximum EUR can be obtained when a horizontal shale gas well has a fracture coefficient of 1.6, Type I reservoir of 18 m thick, optimal horizontal section of 1600 m long, and 20 fractured sections.

The Sichuan Basin is a major target for shale gas exploration in present China because of its rich gas stored in abundant black shales with multiple bed series. For further guidance or reference, field exploration and development practices in the shale reservoirs Upper Ordovician Wufeng–Lower Silurian Longmaxi shale reservoirs were studied in terms of development stages and progress, favorable conditions for shale gas accumulation, bottlenecking issues on theories and technologies related to shale gas development, and so on. The following findings were obtained. (1) Shale with rich organic matters originated from the deep shelf has a good quality and great thickness in the continuous beds. The relatively stable wide buffer zones in synclines (anticlines) provides favorable conditions for shale gas accumulation and preservation with well-developed micro-fractures and overpressure as necessary factors for a great potential of high shale gas productivity. (2) The bottlenecking technical issues restricting the shale gas industrial development in this study area include the following aspects: understandings of rich-organic matter shale sedimentary facies and modes, shale reservoir diagenetic process and evaluation systems, shale gas generation and accumulation mechanism, geophysical logging identification and prediction of shale gas layers, low resource utilization rate, great uncertainty of shale gas development, no technological breakthrough in the exploration of shale gas reservoirs buried deeper than 3500 m. In conclusion, this study area will be the major target for the shale gas exploration and development in China in a rather long period in the future. Keywords: Sichuan Basin, Upper Ordovician Wufeng–Lower Silurian Longmaxi, Shale gas, Exploration and development, Progress, Challenge, Prospect

As an emerging unconventional energy resource, shale gas has great resource potential and developmental prospects. The effective evaluation of geological sweet spots (GSS), engineering sweet spots (ESS) and comprehensive sweet spots (CSS) is one of the main factors for a high-yield scale and economic production of shale gas. Sweet spot evaluation involves a comprehensive analysis based on multiple parameters. Conventional evaluation methods consider relatively simple or single factors. Although the main influencing factors are understood, the influence of different factors is as of yet unknown, and a comprehensive consideration may strongly affect the evaluation results. In this paper, the fuzzy mathematics method is introduced for shale gas sweet spot evaluation. With the help of fuzzy mathematics tools, such as membership function, the objective of comprehensive sweet spots evaluation based on multiple parameters is realized. Additionally, the reliability of the evaluation of sweet spots is improved. Firstly, previous research results are used for reference, and the evaluation factor system of geological and engineering sweet spots of shale gas is systematically analyzed and established. Then, the basic principle of the fuzzy comprehensive evaluation method is briefly introduced, and a geological engineering integrated shale gas sweet spots evaluation method, based on the fuzzy comprehensive evaluation method, is designed and implemented. Finally, the data from HB blocks in the Z shale gas field in China are adopted. According to the evaluation results, the modified method is tested. The results show that the method proposed in this paper can synthesize a number of evaluation indices, quickly and effectively evaluate the GSS, ESS and CSS in the target area, and the results have high rationality and accuracy, which can effectively assist in well-pattern deployment and fracture design.

To further understand shale reservoir characteristics of Wufeng Formation and Longmaxi Formation in the Wuxi area, northeast Chongqing, based on drilling data of Well WX2, and taking the graptolite biostratigraphy as the standard marker of stratigraphic division and comparison, the geochemistry, petrology, reservoir space and properties of organic-rich black shale were well investigated, and its gas-bearing capacity and controlling factors were also analyzed. The result shows that in the Wufeng Formation and Longmaxi Formation of Well WX2, the organic-rich shale is 89.8 m thick and is characterized by good kerogen type, high organic abundance, moderate maturity and favorable hydrocarbon-generation condition, and the graptolite sequence is developed completely and continuously; the organic abundance is influenced by depositional rate, and the slow depositional rate is favorable for accumulation of organic matter in the black graptolite shale; from top to bottom, content of siliceous minerals increases and content of clay minerals decrease, therefore the brittleness increases; the organic-rich siliceous shale and clay siliceous shale are favorable lithofacies for development of shale reservoirs; the nanopore is dominated by the parallel-plate pore with four open sides and has good connectivity; the pore size distribution curve has the multimodal characteristic, and the pore diameter mainly is in the range of 0.42–0.62 nm and the range of 3–5 nm; organic pores and interlayer pores of clay minerals make the greatest contribution to the total pore volume, while pores of brittle minerals have the least contribution; from top to bottom, organic pores gradually increase while interlayer pores of clay minerals gradually decrease; the on-site core gas content exceeds 8 m3/t, and the gas-bearing capacity is jointly controlled by hydrocarbon generation, reservoir and preservation conditions; and the WF2-LM6 biozone of the Katian to the early Aeronian is the high-quality shale reservoir, where the LM1 biozone of the Hirnantian was the best “sweet spot” which is the target of horizontal well drilling. Keywords: Shale gas reservoirs, Graptolite biozone, Micropore, Gas-bearing capacity, Wufeng Formation and Longmaxi Formation, Northeast Chongqing

A large proportion of shale gas is in adsorbed state. However, in the process of shale gas development, adsorbed gas in the shale pores will gradually desorb, which results in the permeability change of shale. This study aims to find out the adsorption and desorption characteristics of shale and whether there exists certain impact of desorption effects on shale permeability by experiments. Isothermal adsorption and desorption curves of shale gas are not coincident and the desorption curves are in hysteresis. Langmuir equation can be used to calculate the shale isothermal adsorption curve and desorption equation can be used to calculate the shale gas isothermal desorption curve. The permeability of shale was measured, respectively, under low pressure and high pressure, with methane and helium as experimental gas. Then, the comparison was made between the results of shale and the similar experiment results of tight gas sandstone. The results are showed as follows: adsorption effects will break the linear relationship between the shale permeability and the reciprocal of average pore pressure under low pressure; under high pressure, when gas desorption enhances, shale permeability significantly increases. Therefore, adsorption effects can impact the shale permeability under low pressure as well as under high pressure, which should be taken into consideration in the shale gas development.

Shale gas has become an important source of natural gas, and a significant portion of gas in the reservoir exists in an adsorbed state. Thus, knowledge of gas adsorption behavior is important to estimate original gas in place and shale gas production. In this paper, shale gas adsorption affecting factors which include temperature, pressure, pore type, and diameters were studied quantitatively and systematically using simplified local density model. It was found that the main affecting factors were temperature, pore type, diameter, and pressure; type I kerogen has the lowest and type III kerogen has the largest excess adsorption gas capacity; due to the interaction energy between gas molecules and pore wall molecules, gas has the highest local density near the pore wall and the lowest density in the middle of the pore under a certain pressure and temperature; the increment of gas density as temperature decreases near the pore wall is significant, but the bulk methane gas density shows a very small change due to temperature change; the ratio of excess adsorption gas to free gas declines and its rate slows down as pressure increases; when the pore diameter increased from 1.5 to 5 nm, the ratio of excess adsorption gas to the total gas amount decreases rapidly; the excess adsorption gas of type III kerogen takes up 89% of the total gas amount with diameter of 1.5 nm and takes 70% of the total gas amount with diameter increases to 5 nm; the excess adsorption gas of type I kerogen takes 71% of the total gas amount with diameter of 1.5 nm and takes up 43% of the total gas amount with diameter increases to 5 nm. Local gas density in different kinds of pores under different conditions also provided a way to calculate gas transport in pores that take adsorption gas into consideration.

As an important unconventional gas resource, shale gas has become an important part of gas production in recent years with the advantage of horizontal well drilling and large-scale multi-stage hydraulic fracturing completion technologies. The shale gas reservoir numerical simulation advances were reviewed and a multi-stage fractured horizontal well numerical simulation was performed to qualitatively modeling the well productivity in over-pressured shale gas reservoir based on actual shale properties and well completion parameters. A single horizontal well model was established on the basis of dual-porosity model and logarithmically spaced grid refinement. A comprehensive comparison and analysis of the initial average gas production, daily gas production, cumulative gas production, adsorbed gas and free gas cumulative production were provided to investigate the influence of matrix permeability, SRV permeability, hydraulic fracture conductivity and half length, SRV size, bottomhole pressure on the well performance. The research shows that for the high matrix permeability (K m > 10 −7 mD) and low SRV permeability (K SRV 10 −7 mD) and medium SRV permeability (0.01 mD 10 −7 mD) and high SRV permeability (K SRV > 0.5 mD), the initial average gas production is mainly controlled by the matrix permeability. When the matrix permeability is lower than 10 −9 mD, the cumulative gas production is too low to be of economic interest. For the matrix permeability (10 −9 mD 10 −5 mD), the matrix permeability has much more impact on cumulative gas production than that of SRV permeability. The daily gas production and cumulative gas production are independent of hydraulic fracture conductivity and half length. The initial gas production of multi-stage fractured horizontal well is also independent of SRV sizes. The SRV size mainly controls the gas production decline characteristic. With the increase of the SRV size, the daily gas production declines slowly. The SRV size determines the cumulative gas production directly. With the increase of the SRV size, the cumulative gas production increases linearly. The bottomhole pressure has a significant impact on cumulative gas production. With the decrease of the bottomhole pressure, the cumulative gas production of 20 years increases linearly.

Isothermal adsorption and desorption experiments under different temperatures were carried out with the Longmaxi Formation shale samples collected from southern Sichuan. The experimental results show that temperature affects the adsorption and desorption capacity of shale, the adsorption capacity of shale decreases with temperature increase. The adsorption curve and desorption curve of shale are not coincident and the thermodynamic reason for the hysteresis of the desorption curve is that the isosteric heat of the shale adsorption process is greater than that of the desorption process. The Langmuir model and desorption model can describe the isothermal adsorption and desorption processes very well, respectively. Isothermal adsorption and desorption curves under different temperatures can be predicted by isosteric heat curves which match the experimental results. Shale gas production is a process of gas desorption and the desorption characteristics directly impact the production of shale gas, so the desorption model should be taken into consideration in the shale gas production forecast and numerical simulation. Key words: shale, temperature, adsorption, desorption, isosteric adsorption heat